Extrusion of peroxide crosslinkable polymer parts

A method for extruding a peroxide crosslinked polymer tube, comprising:

• supplying a mixture to a screw extruder, the mixture comprising:
• a crosslinkable polymer, a crosslinking agent, and a stabilizing agent, wherein the polymer has a crystallite melting point and a crosslinking temperature, and wherein the screw extruder is a conically tapered screw extruder having at least two sections of differing spiral pitch;
• heating the mixture in the screw extruder with an external heating unit to a temperature above the crystallite melting point but below the crosslinking temperature;
• controlling the temperature of the mixture in the screw extruder with the external heating unit and an internal cooling unit;
• continuously feeding the mixture from the screw extruder to an extrusion die, wherein a melting pressure before entry to the extrusion die is approximately 700-1500 bar;
• heating the mixture in the extrusion die above the crosslinking temperature to effect at least a partial crosslinking of the polymer in the extrusion die, wherein the temperature (° C.) of the mixture in the extrusion die is at least 15% above the crosslinking temperature (° C.);
• and
• discharging the mixture from the extrusion die, wherein the degree of crosslinking of the polymer on discharge from the extrusion die is above 60%.